I have no idea what a sailsbury drive is, but I had a good salisbury steak the other day
I've never seen the rpm changer on a machine, but I heard of them. I don't think there was any intelligence to the drive I was thinking of, it was more like a replacement for the manual hand crank to operate the variable speed drive. So it probably counted turns or perhaps relied on some kind of timing trick to estimate where the speed was. Since Bandit used percentage spindle speed commands, you never did command rpm directly with it, S99 was max speed with the motor at full rpm. Now with an rpm changer, I suppose you left the motor running full speed all the time (which was likely an annoyance) and used the S commands to control the rpm changer.
To get good torque on your motor, you could step up the motor size and get a vector drive VFD from Baldor. Encoder feedback is the ticket that makes a vector drive work so well. The vector drive also runs exactly at the commanded rpm. However, I only used floating tap drivers to tap with, but they worked well enough for me to afford a Haas later on
If your Shizouka was like mine, it had about a 2hp motor on it. I bumped that up to a 5hp, and took the v-speed sheaves off, and put in a poly-v belt drive, and a vfd. I should have used a vector drive but didn't know any better at the time. The spindle would stall out in low rpm tapping with 3/8" taps.
Actually, I have two step pulleys with poly-v profile for a high and low range, but the shift has to be done manually, so typically it was left in high range. I'd run that machine as fast as 5000 rpm. If the job warranted a low speed session with more power for tapping, then, I'd opt to run the whole program with the belt in 'low range'.
Incidentally, I chose a spindle motor with a spring brake on it. This was to help stop the spindle from free wheeling when the tool changer impact wrench was doing its thing.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)